Recently, research and development of a current-driven light-emitting element such as an organic EL (electroluminescence) display or an FED have been actively carried out. Particularly, an organic EL display is a display which can emit light at low voltage and with low power consumption, and draws attention as being used for a mobile device such as a mobile phone or a PDA (personal digital assistance).
As an arrangement of a current-driven pixel circuit of such an organic EL display, a circuit arrangement disclosed in Japanese PCT Laid-Open Application No. 514320/2002 (Tokuhyo 2002-514320; published on Oct. 29, 1998: Corresponding to PCT International Publication No. WO/98/48403) is shown in FIG. 25.
A pixel circuit 300 shown in FIG. 25 includes four p-type TFTs (thin film transistors) 360, 365, 370, and 375 and two capacitors 350 and 355, and an organic EL (OLED) 380. The organic EL 380 is a current-driven electro-optic element and serves as a display light source. The TFTs 365 and 375 and the organic EL 380 are serially connected in this order to a path extending from a power supply line 390 to a common cathode (GND line). The capacitor 350 and the switching TFT 360 are serially connected in this order to a path extending from a gate terminal (current control terminal) of the driving TFT (driving transistor) 365 to a data line 310. Further, the switching TFT 370 is connected between the gate terminal of the driving TFT 365 and a drain terminal (current output terminal) of the driving TFT 365, and the capacitor 355 is connected between the gate terminal of the driving TFT 365 and a source terminal (reference potential terminal) of the driving TFT 365. A select line 320 is connected to a gate terminal of the TFT 360. An auto zero line 330 is connected to a gate terminal of the TFT 370. A lighting line 340 is connected to a gate terminal of the TFT 375.
In the pixel circuit 300, a voltage of the auto zero line 330 and a voltage of the lighting line 340 become Low in the first period, and the switching TFTs 370 and 375 are put in an ON state, so that the drain terminal of the driving TFT 365 and the gate terminal of the driving TFT 365 have the same potential. At this time, the driving TFT 365 is put in an ON state, so that a current flows from the driving TFT 365 to the OLED 380.
Further, at this time, a reference voltage is inputted into the data line 310, and a voltage of the select line 320 is made Low, so that an opposite terminal (TFT-360-side terminal) of the capacitor 350 has the reference voltage.
Next, in the second period, a voltage of the lighting line 340 becomes High, putting the TFT 375 in an OFF state.
In this way, a gate potential of the driving TFT 365 gradually increases, and when the gate potential of the driving TFT 365 reaches a value (+VDD−Vth) corresponding to a threshold voltage (−Vth) of the driving TFT 365, the driving TFT 365 is put in an OFF state.
Then, in the third period, a voltage of the auto zero line 330 becomes High, putting the switching TFT 370 in an OFF state. In this way, a difference between a gate potential of the capacitor 350 and a reference potential of the capacitor 350 is stored in the capacitor 350.
That is, when a potential of the data line 310 is the reference potential, the gate potential of the driving TFT 365 is the value (+VDD−Vth) corresponding to the threshold voltage (−Vth) of the driving TFT 365. Moreover, when the potential of the data line 310 changes from the reference potential, a current corresponding to the potential change flows into the driving TFT 365 regardless of the threshold voltage of driving TFT 365.
Accordingly, such a desired potential change is supplied to the data line 310, so that a voltage of the select line 320 becomes High, and the switching TFT 360 is put in an OFF state. In this way, a potential of the gate terminal of the driving TFT 365 is maintained, and a pixel selection period is terminated.
Thus, the use of the pixel circuit shown in FIG. 25 makes it possible to compensate fluctuation in the threshold voltage of the driving TFT 365, thereby supplying a potential of the compensated threshold voltage (desired potential—threshold voltage) to the gate terminal of the driving TFT 365.
Further, as another arrangement of a current-driven pixel circuit of an organic EL display, a circuit arrangement disclosed in Japanese PCT Laid-Open Application No. 529805/2003 (Tokuhyo 2003-529805; published on Oct. 11, 2001: Corresponding to PCT International Publication No. WO01/075852) is shown in FIG. 26.
A pixel circuit Aij shown in FIG. 26 includes three p-type TFTs 30, 32, and 37 and an n-type TFT 33, a capacitor 38, and an organic EL (OLED) 20. The organic EL 20 is a current-driven electro-optic element and serves as a display light source. The TFTs 30 and 33 and the organic EL 20 are serially connected in this order to a path extending from a power supply line 31 to a common cathode (GND line) 34. The switching TFT 32 is disposed between a gate terminal (current control terminal) of the driving TFT 30 and a drain terminal (current output terminal) of the driving TFT 30, and the capacitor 38 is disposed between the gate terminal of the driving TFT 30 and a source terminal (reference potential terminal) of the driving TFT 30. The switching TFT 37 is connected between the drain terminal of the driving TFT 30 and a source wire Sj. A gate wire Gi is connected to a gate terminal of each of the TFTs 32, 37, and 33.
With this arrangement, in a time period (selection period) during which a voltage of the gate wire Gi is Low, the switching TFT 33 is put in an OFF state, and the switching TFTs 32 and 37 are put in an ON state. As a result, a current flows from the power supply line 31 through the driving TFT 30 and the switching TFT 37 to the source wire Sj. When a current value at this time is controlled by a current source of a source driver circuit (not shown) connected to the source wire Sj, a gate voltage of the driving TFT 30 can be set so that an output current value of the driving TFT 30 becomes equal to the current value regulated by the source driver circuit.
Thereafter, a voltage of the gate wire Gi becomes High, so that the TFTs 32 and 37 are put in an OFF state, and the gate voltage of the driving TFT 30 is retained. Further, the TFT 33 is put in an ON state, so that the current value set in the selection period is outputted from the driving TFT 30 to the organic EL (OLED) 20.
Thus, the use of the pixel circuit shown in FIG. 26, regardless of fluctuation in threshold voltage of the driving TFT 30 or fluctuation in mobility of the driving TFT 30, makes it possible to set a gate potential of the driving TFT 30 so that the output current value of the driving TFT 30 becomes equal to the current value supplied from the current source of the source driver circuit.
As described above, the use of the pixel circuit shown in FIG. 25 makes it possible to compensate the fluctuation of the threshold voltage of the driving TFT 365. However, with the pixel circuit arrangement of FIG. 25, it takes several tens of microseconds (Vs) for the driving TFT 365 to shift from an ON state to an OFF state, and the reference potential must be retained in the data line 310 during this period. This increases the duration of selection period per pixel, and thus results in display of less number of pixels.
Further, with the pixel circuit arrangement of FIG. 26, it is possible to compensate the fluctuation in the threshold voltage of the driving TFT 30 and the fluctuation in mobility of the driving TFT 30. However, the foregoing problem occurs more conspicuously.
More specifically, also in the pixel circuit of FIG. 26, the source wire Sj has a floating capacitance. Therefore, the pixel circuit of FIG. 26 is controlled so that a desired current flows from the driving TFT 30 to the source driver circuit. Therefore, when a value of the desired current is small, it takes several hundred microseconds (μs) or more only to charge the floating capacitance.
As a result, a selection period per pixel is lengthened. This raises a problem that the number of pixels which can be displayed decreases accordingly.